Linux Sheep It Client Doesn't Recognize Nvidia GPU
The easy(ish) answer is that Cuda isn't actually installed by default on every platform.  Blender actually compiles in CUDA libs statically AFIK, but in other programs you will need to install the CUDA toolkit.

https://community.linuxmint.com/software/view/nvidia-cuda-toolkit

Additionally, you can actually install the version from the NVidia site, but it's a bit more compilcated: 

https://www.reddit.com/r/deepdream/comments/3cd1yf/howto_install_on_ubuntulinux_mint_including_cuda/